Photoablation eximer laser keratectomy is a ophthalmologic technique which employs an 193 nm excimer laser as a surgical tool to ablate or remove a precise amount of tissue from the anterior corneal surface of the eye. In addition to its usual application in correcting refractive errors (e.g. myopia, hyperopia and astigmatism) by altering the curvature of the cornea, this technique has more recently been applied, with success, in removing opacities and irregularities from the anterior corneal surface.
The application of eximer lasers in photoablation procedures have been described in the medical literature. See, for example, Sher, N. A. (1991 ) Arch. Ophthal., Vol. 109 , pages 491-498 ; Zabel, R. W. et al. (1990 ) Refrac. Corn. Surg., Vol. 6, pages 329-334 ; Steinert, R. F. et al, ibid, page 352 ; Gaster, R. N. et al. (1989 ) Invest. Ophthal. Vis. Sci., Vol. 30 , pages 90-98; and Tuft, S. J., ibid, page 1769-1777. Typically, a far ultraviolet argon fluoride laser, emitting at 193 nm, is used in clinical procedures because of its minimal tissue interaction, ablative efficiency and ease of control of ablation depth. Moreover, irradiation at 193 nm shows less mutagenic potential in comparison to longer ultraviolet wavelengths. The laser, fitted with a series of apertures of varying diameter and shapes, is preprogranmmed to deliver a series of pulses of a given duration and energy fluence settings. In general, eye movements are minimizd during the ablating process and the eye is held with the visual axis fixated under the center of the laser beam.
The extent and depth of the ablation depends on a number of variables which include aperture, shape and diameter, laser energy fluence (mJ/cm.sup.2), duration of irradiation (nanoseconds), pulse rate (Hz) and number of pulses. Other factors would include intraoperative epithelium and corneal stromal drying effluent removal. Eximer laser ablation of the anterior corneal lamellar tissue, in general, leaves behind a smooth surface that enables reepithelialization, a clearer cornea, and an appropriate refractive surface.
In certain situations modulators are used during the procedure. As defined herein, the term "modulator" refers to a substance which, when applied to tissue, is capable of absorbing UV irradiation and modulating the degree of tissue ablation. Modulators are generally used as adjuncts to promote photoablative smoothing of irregular corneal surfaces and to protect adjacent corneal tissue where ablation is not desired. Examples which would benefit from the use of modulators include removal of corneal scars and opacities, often accompanied with an irregular or rough epithelial surface, due to post-infectious and post-traumatic causes, including herpes simplex virus, dystrophies (e.g. Salzmanns and Reis Buckler's syndrome), recurrent erosions and band keratopathy. Also, several types of corneal pathologies ablate more quickly than others, and this differential ablation may lead to increased irregularity of the corneal surface following ablation.
A number of photoablation modulators have been reported in the literature. See, for example, Sher, N. A. (1991), supra; Steinert (1990), supra; Kornmehl, E. W. et al. (1991) Investigative Ophthalmology & Visual Science, Vol. 31(4), Page 245, Abstract no. 1203; and Steinert, R. F. in "Eximer Laser Phototherapeutic Keratectomy: Strategies and Representative Cases," 17th Cornea Research Conference, Sep. 19-21, 1991, Eye Research Institute and Massachusetts Eye and Ear Infirmary. Examples of known modulators include viscous aqueous solutions of methylcellulose, dextran 70, sodium carboxymethylcellulose and hydroxypropylmethylcellulose 2910 as well as 0.9% saline.
In general, conventional modulators suffer from a number of deficiencies which preclude their broader use in photoablation procedures. For example, modulators (1) are difficult to apply smoothly on the corneal surgical bed; (2) are susceptible to drying and rippling from the air flow from the effluent remover; (3) do not adequately absorb at 193 nm; and/or (4) do not adequately promote a smoother ablated corneal surface relative to a control situation (no modulator) because the modulator ablates at a different rate than corneal tissue. Accordingly, there is a need in the art for photoablation modulators which avoid one or more of the aforementioned deficiencies.